The present invention relates generally to the field of data storage devices, and in particular to runout calibration for data storage devices using a rotatable disc.
Rotatable discs (or disks), as the term is used herein, include hard discs, high-capacity floppies and optical discs--including read only compact discs (CD) and digital versatile discs (DVD), as well as their writable counterparts (e.g., CD-R, CD-RW and DVD-RAM). In data storage devices employing rotatable discs (e.g., "disc drive systems"), information is read from and/or written to such a disc (while it is rotating) by a transducing data head supported adjacent the disc surface. The data head type is determined by the subject disc technology; e.g., it may be electromagnetic to access magnetic discs, optical to access CDs and DVDs, etc.
Rotatable discs (or "disks") store data in tracks. For example, magnetic discs store data in a series of concentric tracks while CDs store data in a single, spiral track (like a phonograph record) that circumnavigates the disc thousands of times (e.g., over 20,000) as it gradually moves away from the center. For ease of discussion, each rotation of the spiral track is referred to herein as a track.
A conventional disc drive system typically includes a positioning servo loop or, more generally, a servo system (containing one or more servo loops) to position and maintain the data head precisely over a selected track of the disc. The operation of maintaining the head over a desired track (i.e., normal play) is known as "track following" and that of moving the data head between tracks is known as track "accessing," "searching" or "seeking." The ability of a data head to stay locked on a single track is complicated by the large number of tracks and the high speed at which the discs rotate. Staying locked on a single track is further complicated by any irregularities in the track shape.
Many discs have some type of irregularity which causes a track's center to be offset from a disc's axis of rotation (disc "eccentricity"). This can be caused by a number of factors, including irregularities in a disc's manufacture. This eccentricity results in "disc runout." Disc runout is the lateral displacement of a track which, as the underlying disc rotates, appears sinusoidal from the vantage point of a data head. Conventional servo systems performing track-following operations attempt to compensate for this phenomenon.
FIG. 1 illustrates a conventional servo loop 10 for use in a disc drive system, which includes a servo controller 14 coupled to drive mechanics 18 (e.g., servo(s) used to position a data head) via an amp 16 and a feedback loop 19. Signals associated with a track follow operation are also included in this figure. "XI" is a physical position input, which includes a near-sinusoidal component representing disc runout. "XO" is a physical position output, which represents the dynamic position of a data head relative to a rotatable disc.
In the presence of disc runout, XI may represent the movement of many tracks near-sinusoidally at the disc rotational frequency. The position output, XO, must follow this multi-track input within a fraction of the track pitch to avoid excessive position error and thus data errors. In an attempt to accomplish this track follow requirement, a summing junction 12 compares position input XI and position output XO to produce a physical position error XE. Position error XE must be less than some prescribed value over the entire time of one full disc revolution to achieve accurate track follow during data read/write operations.
The position error XE is processed and fed into a servo controller 14, which produces a controller output drive signal XC, which typically approximates the acceleration of the drive mechanics. Controller output XC is in turn passed through an amplifier 16 and fed into drive mechanics 18 as signal XOdd to drive the mechanics of the servo loop.
Ideally, drive signal XC should correct any positional error of the data head. However, as disc rotational speeds increase and track spacings get smaller in the presence of large runout, it becomes more difficult for a conventional analog or digital servo controller to maintain acceptably small position errors (XE) during all conditions, particularly following a track searching operation. This is due, in part, to the fact that there is a limit to the responsiveness of such servo systems if instabilities at certain resonant frequencies are to be avoided. Moreover, position loop bandwidth limitations in conventional servo systems also affect responsiveness, especially where the disc runout can reach or exceed several hundred tracks.
In other words, where significant eccentricity exists, conventional servo systems can have difficulty following certain deviations of the tracks during a track following operation resulting in reading and/or writing errors. This problem increases as the number of tracks on a disc increase and/or the rotational speed of the disc increases in the presence of large runout.
Like track following, track searching is also detrimentally affected by disc runout. In a track search, drive mechanics of the servo system try to move a data head quickly and reliably to any desired track in order to begin data operations. This operation may be broken down into two steps: (1) high-speed search and (2) low-speed settling. In a high-speed search, the servo system tries to accelerate a data head to a very high velocity to minimize search time. Feedback typically comes from the relative velocity between the motion of the head and the oscillating tracks (due to runout). There may be a finite limit to the magnitude of the relative velocity which can be reliably measured. If the runout is sufficiently severe, this limit may be exceeded resulting in search failure.
Low-speed settling means grabbing onto a track (hopefully the target track) using the position servo system and centering the data head quickly. With a large disc runout, the relative speed of the oscillating tracks may be many times larger than a desired "safe" locking speed (which the servo system is trying to maintain as it attempts to converge on the target). Settling problems may include (a) overshoot within the target track which would increase time-to-data, (b) skipping one or more tracks away from the target--requiring a re-search, or (c) skidding forever and timing out or possibly requiring a drive reset.
Therefore, it would be desirable to provide a method and system for accurately and reliably compensating for disc runout. It would be further desirable to compensate for disc runout during both track following and track searching operations.
In one aspect, it would be desirable to provide such compensation while minimizing signal noise, thereby enhancing the accuracy of the system.